Utilizing high-precision geochronology to evaluate the rates of geologic processes

Abstract:

High-precision U-Pb zircon and Sm-Nd garnet isotopic ages are used to investigate the rates of geologic processes over timespans of millions of years and spatial scales ranging from mountain belts (km) to thin-sections (microns). New techniques are presented to improve the precision and spatial resolution of the isotopic data used for calculating garnet Sm-Nd ages. Established analytical methods are used to determine U-Pb ages for complex zircon. These techniques are used to address: (1) a tectonic problem in the North Cascades crystalline core; and (2) to evaluate the fundamental process of chemical equilibrium in a garnet-bearing schist from the central Appalachians. Previously-published whole-grain detrital zircon ages for the Swakane Gneiss have been interpreted to reflect post ca. 73 Ma deposition of the protolith sediments and used to propose rapid loading of the Swakane protolith to ca. 35 km in < 5 m.yr. However, Swakane Gneiss zircons have polyphase growth zoning and preserve a complex depositional and metamorphic history. The rapid loading model is tested by correlating new garnet Sm-Nd ages with spot ages of metamorphic zircon overgrowths to establish the timing of metamorphism. I also construct P-T-t paths for the depositional and loading history of the Swakane Gneiss. Garnet-rock Sm-Nd isochrons of 73.5±1.2, 71.3±2.8, and 65.8±0.7 Ma, document the timing of garnet growth. Homogeneous metamorphic zircon rims have high U/Th (> 5) and define an array of concordant U-Pb ages from 75 to 63 Ma. P-T-t paths for six Swakane Gneiss samples compliment these new ages by documenting a metamorphic history involving: (1) loading of the protolith sediments to 24-33 km, likely via overthrusting of arc rocks; (2) heating to peak temperatures of 650-710°C; and (3) decompression and cooling during exhumation. The new P-T-t dataset documents that regional metamorphism of the Swakane Gneiss protolith occurred during thrust loading and subsequent heating between 75 and 63 Ma. A new model for pre-75 Ma deposition and 75-63 Ma metamorphism of the Swakane Gneiss is proposed that does not require rapid loading rates. To evaluate the process of chemical equilibrium in rocks, I test the idea that Mn contents and high-precision Sm-Nd ages of compositionally-equivalent garnet crystal segments should correlate from garnet to garnet throughout an equilibrium volume of metamorphic rock from Townshend Dam, VT. Major element zoning in garnet is concentric, with Mn-rich cores and Mn-poor rims. Garnets are HREE-enriched overall and show little-to-no zoning except for increased MREE and HREE near the rims. Similar garnet compositions and zoning profiles throughout the sample suggest that garnet growth occurred at the same P-T-X-M conditions over the sample volume. Garnet-rock isochrons calculated for 35 compositionally-specific garnet crystal segments range from 383.3±7.4 Ma to 374.9±1.8 Ma. Sampled Mn contents and Sm-Nd age uncertainties make it difficult to evaluate the correlation of Mn and age for each compositionally-equivalent crystal segments throughout the sample volume. However, 7 garnet cores, 14 mantle zones, and 8 rims, define three distinct multi-point isochrons of 380.5 ± 2.0 Ma, 377.4 ± 1.2 Ma, and 376.6±0.9 Ma, respectively, yielding a garnet growth duration of 3.9±2.2 m.yr. These three composite Mn-age zones make up a Mn vs. age curve that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Grouped isotopic ages and Mn contents suggest that major element and isotopic equilibrium were generally maintained throughout the short duration of garnet growth in the sample. However, a detailed correlation of Mn and garnet age is precluded by the short duration of garnet growth in this sample.

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